In a continuously measuring mass spectrometer a small sample of gas, or a gas mixture, is introduced into a vacuum chamber in which the gas is ionized and accelerated to form an ion stream. The ion stream then moves into a magnetic field; the magnitude of the magnetic field determines a characteristic path of motion for each ion as a function of its charge/mass ratio e/m. An ion collector, positioned in the path of motion, effectively measures the ion current and generates an output which is proportional to the concentration of the particular gas one is interested in, in the gas or mixture of gases being measured.
For a given ion concentration, a maximum current output is generated at the ion collector, and is maintained constant for a given length of time, only as long as the ion collector is located accurately in the correct path of motion of the ions. In any mass spectrometer, however, there are a number of factors that may cause effective drift in the required accurate alignment between the ion collector and the path of ion motion (the ion flight path). Thus, during operation an initial accurate adjustment of the instrument to produce a maximum signal, i.e. the stabilization of the peak output position, may be disturbed. Such drift may be occasioned by changes in the acceleration voltage or in the magnetic field; on the other hand, geometric parameters may also become changed as by expansion or contraction of parts of the instrument due to temperature changes.
West German patent publication No. 2134739 and U.S. Pat. No. 3,648,047 describe control arrangements for maintaining a relatively constant level of sensitivity for a mass spectrometer which may be changed by aging or by influences of the surroundings. In those arrangements, the outputs from a plurality of ion collectors, used for differing gas components, are summed up, and the resulting summation signal is compared with a reference signal. As a function of the result of the comparison, an error signal is generated; that error signal, which is used to change the degree of ionization or the number of the ions arriving at the collectors, is proportional to the deviation of the summation signal from the reference signal. A similar stabilization based on a summation or compound signal may be effected by controlling the gain of an amplifier in a circuit which follows the ion collectors, as is suggested in West German Pat. No. 2037698.
These stabilization methods assume the availability of several ion collectors in the mass spectrometer; moreover, they are usable only for the elimination of disturbances which influence all of the collectors in essentially the same manner. They react effectively with respect to those disturbances, such as the plugging of a capillary input, which influence the amplitudes of all of the collector signals, or of the peak signals, but will not necessarily re-align the system with respect to variations in the correct path of motion of the ions. Furthermore, they do not take into account that a given disturbance, such as a change of the acceleration voltage or a geometric variation due to a temperature change, may act differently upon the various collector signals. In particular, the operating peaks of the individual collectors, comprising the collector currents as functions of the acceleration voltage, may have different widths; thus, the current reductions caused by a given disturbance may be appreciably different at the individual collectors.